I would like to find a vinyl bag with a drawstring closure to cover my Brooks saddle while transporting the bike on the car. I want to protect the saddle from rain and sun. Does anybody know where I could get one? I use a plastic grocery bag for overnight in camp, but I'm looking for something heavier that will hold up at highway speeds and will also protect against UV rays. Thanks for your ideas.

Yeah, I can't get excited about paying $8 for the Brooks cover when it is not waterproof. I figure the size and type of bag I'm looking for would have many applications and would be easy to find, but all I've found so far are the dry bags like this:

I just use a plastic bag. It's works well and I don't have to worry about losing it cause it's free. But if you feel like spending money, the Velox saddle cover looks like it would be good choice.

It's cheaper than the one that Brooks makes and it's actually waterproof. Plus, it looks like it even fits better. (Disclaimer: I don't have personal experience with this cover, just going by the info on this website: http://www.wallbike.com/brooks/veloxseatcover.html)

http://www.truantdesigns.com/Seat%20Covers.htm
These are pretty heavy duty. Fabric feels like canvas with a waterproof coating on the inside. Should stay in place even at highway speeds, but you can tuck the sides of the cover between the saddle leather and saddle rails for a more secure fit. They have a strap to attach to seatpost so you never lose it.

I've always just used a plastic grocery bag, but if you want to go fancy, your local Dollar store might have something. I've picked up several different sorts of bags and things there which have come in handy for randonneuring and touring.

Dry Ride cover from REI for $12. I have one for my Brooks saddle and so far so good. I tuck this into my helmet rain cover, roll them up and store them either in my bar bag or trunk rack, depending. The saddle is probably the fanciest thing I have on the bike, so I justified a rare splurge for this item.